1. Field of the Invention
The present invention relates in general to a transistor control component and, in particular, a gate biasing scheme for improving the low frequency distortion characteristics of the control component and the attendant power handling capabilities thereof. More particularly, the present invention relates to a low distortion MESFET control component particularly for baseband applications.
2. Background Discussion
In microwave integrated circuits, devices such as field effect transistors are very commonly used. For example, GaAs MESFETs are presently widely used in broadband control applications, functioning as, for example, switches and variable attenuators. Because the control port (gate) is isolated from the RF ports (source and drain), the MESFET consumes little prime power in either of its two operating states. In addition, MESFETs are capable of sub-nanosecond switching and the devices are compatible with conventional MMIC processing. Due to this, MESFETs are increasingly employed in baseband applications such as KHz to GHz sweep oscillators, high speed A/D converters, digital step attenuators and solid state relay replacements.
However, at low frequencies, MESFETs exhibit increased distortions and can handle about 10 dB less power than at higher frequencies. This phenomenon has been documented experimentally and confirmed by switches described in the following relatively recent articles; M. J. Schindler and A. Morris, "DC-40 GHz and 20-40 GHz MMIC SPDT Switches," IEEE Trans. Electron Devices ED-34, Dec. 1987, pp. 2595-2602; M. J. Schindler, M. E. Miller, and K. M. Simon, "DC-20 GHz N X M Passive Switches," IEEE Trans. Microwave Theory and Tech., MTT-36, Dec. 1988, pp. 1604-1613; and R. J. Gutmann, N. Jain, M. Schindler, M. Miller, and K. Simon, "Comments on GaAs MESFET Baseband-to-Microwave Passive Switches," IEEE Trans, Microwave Theory and Tech., MTT-37, July 1989, pp. 1154-1155.
Because these MESFET devices are becoming so widely used, there is a real need to improve the low frequency power handling characteristics of these devices. The low frequency power handling problems occur when the gate ceases to be strongly coupled to the RF path. This takes place at low frequencies when the capacitive reactance from the device-to-gate node is significantly increased and the regularly floating gate becomes AC grounded. Unsuccessful attempts have been made at curing these low frequency power handling problems.
One widely accepted attempt at curing the low frequency power handling problems includes gate bias circuitry which consists of a 2-5K ohm resistor at the gate port of the MESFET. The resistor somewhat improves distortion performance, however, these conventionally biased MESFETs still incur low frequency power handling problems. Further attempts have been made to improve low frequency power handling problems by increasing the value of the resistor tenfold from approximately 2-5K ohm to approximately 25-50K ohm. By increasing the resistance tenfold, the frequencies at which distortion performance is improved is reduced tenfold, however, switching speed is also increased tenfold. Thus, there is a trade-off between improved distortion performance and slower switching speeds.
The low frequency power-handling degradation of a conventionally biased control MESFET is due to reduced AC signal coupling to the gate. At high frequencies (above 100 MHz) the capacitive reactance of the active device-to-gate node is much lower than the gate biasing resistance so that the gate is floating. However, at low frequencies the capacitive reactance is much higher than the gate biasing resistance, and the gate becomes AC grounded thereby degrading the strong coupling path to the RF ports.
In the conducting state, reduced AC signal coupling to the gate at low frequencies results in early power saturation and gate current injection. At low frequencies, the channel voltage immediately below the gate in the channel fails to couple on to the gate. If the RF voltage is in the positive half-cycle the channel becomes more positive at the RF source end with respect to the gate. Thus, the channel begins to close. If the RF voltage is in the negative half-cycle, the gate becomes more positive with respect to the channel and forward current injection occurs. Neither situation is desirable.
In the non-conducting state, reduced AC signal coupling to the gate at low frequencies results in lower AC voltages at which the device comes out of pinch-off and goes into avalanche breakdown. When the voltage between source-gate or drain-gate falls below pinch-off, opening of the channel occurs during part of the RF cycle. At high frequencies, the gate voltage is between the source and drain voltages. If the source is grounded and the drain voltage swings by V volts, the gate will swing around the bias voltage by about V/2. Thus the gate-to-drain voltage will swing from V.sub.bias +V/2 to V.sub.bias -V/2. At low frequencies, the voltage fails to couple to the gate, and therefore the gate-to-drain voltage will swing from V.sub.bias +v to V.sub.bias -v. This causes opening of the channel earlier than at high frequencies. Similarly junction breakdown occurs prematurely.